The lifting surfaces of an aircraft, mainly the wings and the stabilizers, comprise control surfaces (flaps, ailerons, slats and spoilers in the wings and rudders and elevators in the stabilizers) that are movable parts capable of exerting various effects on the flight of said aircraft. The commonest configuration of these control surfaces is that of a simple hinge, in such a way that the control surface rotates relative to the lifting surface on which it is arranged.
Advanced designs of control surfaces for aircraft lifting surfaces are known, called double-hinged, in which the control surfaces comprise two surfaces, a primary control surface and a secondary control surface, movable relative to one another and relative to the lifting surface of the aircraft.
Thus, one of these designs of double-hinged control surfaces, called tabs, which are usually positioned on the stabilizers of an aircraft, comprise two movable surfaces, in which the rotation of the secondary control surface is in the opposite sense to the rotation of the primary control surface, in such a way as to reduce the hinge moment produced by the aerodynamic loads of the aircraft, with a consequent reduction in the force required for moving the control surface, since the rotation of the primary control surface assists the rotation of the secondary control surface. The problem that arises with such control surfaces is that they do not allow an increase in the control power, i.e. the aerodynamic force produced by the control surface, in fact they reduce it as they reduce the effective curvature of the airfoil.
Configurations of double-hinged control surfaces for lifting surfaces of aircraft are also known in which the rotation of the primary and secondary control surfaces is effected in the same sense. These control surfaces are usually of full-span configuration, with the control surface including the whole span of the lifting surface, i.e. the secondary control surface includes the whole span of the primary control surface. This configuration gives rise to problems of stalling of the lifting surface when the angle of attack thereon is high, as well as problems resulting from the lack of stiffness of the secondary element of the control surface.
Configurations of double-hinged aircraft control surfaces are also known in which the rotation of the primary and secondary control surfaces takes place in the same sense, and having partial span, with the secondary control surface only partially occupying the span of the primary control surface. In these cases, actuation of the primary control surface is effected by means of actuators normally located behind the spar of the lifting surface. The problem arising with this configuration, which is for example that used for the rudder of the Boeing 777, is that the jump in effective curvature on deflection of the secondary surface produces a vortex of separated airflow that increases the aerodynamic drag of the control surface.
The present invention aims to solve the shortcomings that have just been outlined.